Deuterostomes represent one of the two major superphyla within the animal kingdom, Bilateria, which includes animals with bilateral symmetry and three distinct tissue layers. This classification is based on a shared pattern of development that occurs during the earliest stages of life, not the adult body plan. The term is derived from Greek, meaning “second mouth,” referring to the unique sequence in which the digestive tract forms. Deuterostomes are a diverse group, ranging from simple marine organisms to the most complex animals, including all vertebrates.
The Embryonic Blueprint: Blastopore Fate
The fundamental distinction between deuterostomes and protostomes centers on the fate of the blastopore, the first opening that forms during gastrulation. Gastrulation is the process where the single-layered blastula reorganizes into a multilayered structure, establishing the three primary germ layers. In deuterostomes, this initial indentation develops into the animal’s anus, marking the posterior end of the digestive tract.
The mouth subsequently forms later in development at a secondary opening created at the opposite end of the embryo. This developmental sequence defines the superphylum Deuterostomia. By contrast, in protostomes, the blastopore develops directly into the mouth (“first mouth”). The anus then forms secondarily at a different location.
This difference in digestive opening formation highlights a deep evolutionary split within the animal kingdom. The presence of a hollow, dorsal nerve cord in many deuterostomes also contrasts with the solid, ventral nerve cord typical of protostomes. The entire body plan is organized around this defining embryonic moment, linking animals that might otherwise appear vastly different as adults.
Other Distinguishing Developmental Traits
Beyond the fate of the blastopore, deuterostomes possess two other developmental characteristics that set them apart from protostomes, starting with cell division. Deuterostomes exhibit radial cleavage, where division planes are oriented parallel or perpendicular to the main axis of the egg. This aligns the cells directly atop one another, creating a symmetrical arrangement. Protostomes typically undergo spiral cleavage, where the cell divisions are oblique, causing the cells to be offset.
Deuterostome cleavage is also indeterminate, meaning the fate of the early embryonic cells is not fixed immediately after division. If the first few cells (blastomeres) are separated, each retains the capacity to develop into a complete organism. This regulative development is why identical twins can form in humans and other deuterostomes. In contrast, protostomes typically have determinate cleavage, where the fate of each cell is set very early, and separating them results in incomplete embryos.
The third major developmental difference is the method by which the coelom (the main body cavity) forms, known as enterocoelous formation. In this process, the coelom originates from outpocketings that pinch off from the archenteron (the primitive gut). These pouches expand and fuse to form the mesoderm layer and the coelomic cavity. Protostomes form their coelom through schizocoely, where the mesoderm begins as a solid mass of cells that later splits to create the coelomic space.
Major Phyla Classified as Deuterostomes
The superphylum Deuterostomia is composed of three phyla: Echinodermata, Hemichordata, and Chordata. Echinoderms, marine invertebrates, include sea stars, sea urchins, and sea cucumbers. While their adult forms display radial symmetry, their larval stages retain the bilateral symmetry shared by all deuterostomes.
Hemichordates, often called acorn worms, live in marine sediments and possess gill slits similar to those found in chordates. This phylum is considered an evolutionary link between the echinoderms and the chordates. The third and most recognizable phylum is Chordata, which includes all vertebrates: fish, amphibians, reptiles, birds, and mammals, including humans.
Chordates are characterized by having a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development. The presence of these traits, along with the shared embryonic pattern, confirms the deep evolutionary relationship among these disparate animals. These phyla demonstrate the vast range of life forms that share a common developmental history defined by the “second mouth” formation.